1. Field of the Invention
The present invention relates generally to an NB-TDD (Narrow Band Time Division Duplexing) CDMA (Code Division Multiple Access) communication system, and in particular, to an apparatus and method for controlling transmission power upon the occurrence of a transmission pause.
2. Description of the Related Art
In general, a 3rd generation mobile communication system may be classified as an FDD (Frequency Division Duplexing) communication system which distinguishes uplink transmission from downlink transmission based on frequency, and as a WB/NB-TDD (Wide Band/Narrow Band Time Division Duplexing) communication system which separates uplink transmission from downlink transmission based on time. The WB-TDD communication system and the FDD communication system use a chip rate of 3.84 MHz, while the NB-TDD communication system uses a chip rate of 1.28 MHz.
In the NB-TDD communication system, communication between a UE (User Equipment) and a Node B is performed through a channel allocated between them. As data is discontinuously (or burstly) transmitted through the channel on a time division basis, long transmission pauses may occur. Thus, when an uplink or downlink transmission signal is transmitted after each transmission pause, its initial transmission power may not be at a proper level due to the transmission pause.
As stated above, the 3rd generation asynchronous mobile communication system employing a time division technique includes the WB-TDD system and the NB-TDD system. A main difference between the WB-TDD system and the NB-TDD system lies in a chip rate: the WB-TDD system uses a chip rate of 3.84 MHz, while the NB-TDD system uses a chip rate of 1.28 MHz. Further, the WB-TDD system and the NB-TDD system use an uplink/downlink transmission power control method for maximally controlling interferences between the UEs in a Node B and interferences to another Node B. A difference in a transmission power control method between the WB-TDD system and the NB-TDD system will be described therein below.
First, a description will be made of a transmission power control method for the WB-TDD communication system.
The WB-TDD communication system uses an open loop power control method for uplink transmission power control, and a closed-loop power control method for downlink transmission power control. In the open loop power control method used for controlling uplink transmission power of a UE in the WB-TDD communication system, the UE measures a propagation loss of a primary common control physical channel (P-CCPCH) transmitted from a Node B, and properly controls its uplink transmission power based on the measured propagation loss so that the Node B can correctly receive a channel signal transmitted by the UE. Here, the P-CCPCH is a channel for transmitting Node B information and system information (SI) to the UEs in the Node B. The P-CCPCH is transmitted at constant transmission power, and a transmission power level of the P-CCPCH is broadcast to the UEs in the Node B. The UE can measure the propagation loss from the Node B, using the broadcast information on the transmission power level of the P-CCPCH. Further, in the closed-loop power control method used for controlling downlink transmission power of the Node B in the WB-TDD communication system, the UE receives its signal from the Node B, and then measures a level (or a signal-to-interference ratio (SIR)) of the received signal. If the level of the signal received from the Node B is less than a predefined threshold, the UE transmits a TPC (Transmission Power Control) command ordering the Node B to increase its transmission power. In contrast, if the level of the signal received from the Node B is higher than or equal to the threshold, the UE transmits a TPC command ordering the Node B to decrease its transmission power. The Node B then controls downlink transmission power based on the TPC command received from the UE so that the transmission power received by the UE may have a constant level. This method is called the “closed-loop power control method”. In other words, the WB-TDD communication system, as stated above, uses the closed-loop power control method for the downlink transmission power control and the open loop power control method for the uplink transmission power control.
However, unlike the WB-TDD communication system, the 3GPP (3rd Generation Partnership Project) TSG (Technical Special Group) RAN (Radio Access Network) TR (Technical Report) specifies that the NB-TDD communication system use the closed-loop power control method for both the uplink transmission power control and the downlink transmission power control. A main difference between the open loop power control method and the closed-loop power control method consists in whether or not to transmit the TPC command. In the WB-TDD communication system, since the closed-loop power control method is used only for the downlink transmission, the TPC command is transmitted through only the uplink. However, in the NB-TDD communication system, since the closed-loop power control method is used for both the uplink and downlink transmissions, the TPC command is transmitted through both the uplink and the downlink.
Next, an uplink/downlink transmission power control method for the NB-TDD system will be described herein below with reference to FIG. 1.
FIG. 1 illustrates a structure of a subframe for an NB-TDD communication system. Referring to FIG. 1, one subframe has a length of 5 ms, and 2 subframes constitute one frame. Thus, the frame has a length of 10 ms, and becomes a radio frame, a basic radio transmission unit, in the 3rd generation mobile communication system. The 10 ms-radio frame becomes a basic radio transmission unit commonly used in the FDD, WB-TDD and NB-TDD communication systems.
The subframe is comprised of 7 time slots TS0–TS6, a downlink pilot time slot (DwPTS, or downlink pilot channel (DwPCH)) 102, an uplink pilot time slot (UpPTS, or uplink pilot channel (UpPCH)) 104, and a guard period (GP) 103. Of the time slots TS0–TS6, time slots TS0 and TS4–TS6 represented by down arrows are downlink time slots transmitted from the Node B to the UE, while time slots TS1–TS3 represented by up arrows are uplink time slots transmitted from the UE to the Node B. Since the NB-TDD communication system switches from the uplink transmission to the downlink transmission and vice versa in a time slot unit, it allocates the time slots of the subframe for the uplink transmission and the downlink transmission according to the following rules.
In the subframe structure, the first time slot (TS0) 101 must be fixedly used for downlink transmission, and the DwPTS 102 is a period for which the Node B transmits a predetermined code sequence to the UE so that the UE can acquire downlink synchronization. The UpPTS 104 is a period for which the UE transmits a predetermined code sequence to the Node B for uplink synchronization. Further, a switching point (SP) 110 represents a point where the downlink transmission is switched. The GP 103 is a non-transmission period established to prevent interference between the DwPTS 102 and the UpPTS 104 due to a possible overlap between them. A switching point 111 is a time point for separating uplink transmission from downlink transmission of the time slots other than the first time slot (TS0) 101. Here, the switching point 111 is variably set up such that the number of uplink slots is larger than the number of downlink slots when there is a greater amount of the uplink transmission data, while the number of downlink slots is larger than the number of uplink slots when there is a greater amount of the downlink transmission data.
FIG. 2 illustrates a method for allocating uplink/downlink channels to UEs by a Node B in the NB-TDD communication system. For simplicity, it will be assumed in FIG. 2 that one Node B allocates an uplink/downlink channel to one UE. Referring to FIG. 2, there are shown a 1st radio frame 201, a 2nd radio frame 202, a (K−1)th ratio frame 203 and a Kth radio frame 204. Each radio frame is comprised of two subframes. That is, the 1st radio frame 201 is comprised of a 1st subframe 211 and a 2nd subframe 212; the 2nd radio frame 202 is comprised of a 1st subframe 221 and a 2nd subframe 222; the (K−1)th radio frame 203 is comprised of a 1st subframe 231 and a 2nd subframe 232; and the Kth radio frame 204 is comprised of a 1st subframe 241 and a 2nd subframe 242.
In FIG. 2, if the UE sends a channel allocation request to the Node B or the Node B is required to allocate a channel to the UE, the Node B informs the UE of such resources as radio frame, subframe, time slot and channelization code to be used for the uplink transmission and such resources as radio frame, subframe, time slot and channelization code to be used for the downlink transmission. Of the resources used for the channel allocation, the channelization code is an orthogonal code. For downlink transmission, the orthogonal code serves to distinguish a downlink transport channel allocated to a specific UE from the downlink channels allocated to the other UEs using the same time slot by allocating different orthogonal codes to the respective UEs. For uplink transmission, the Node B uses the orthogonal codes in identifying the different UEs using the same time slot. Of course, for the uplink/downlink transmission through the orthogonal code, two or more different orthogonal codes can be allocated to the same UE to increase a data rate of the downlink transmission to the UE and the interference uplink transmission from the UE. Further, an OVSF (Orthogonal Variable Spreading Factor) code used in the 3rd generation asynchronous mobile communication systems is typically used for the channelization code used in the NB-TDD communication system. Here, the OVSF code is characterized in that its length is variable according to a spreading factor (SF) for data spreading. For example, if the spreading factor is 4 (SF=4), a data band is expanded 4 times. In this case, a length of the channelization code in use becomes 4, and 4 available (SF=4) channelization codes are generated. Data spreading factors used in the NB-TDD communication system are 1, 2, 4, 8 and 16, and as the spreading factor increases more and more, a data rate of the transmission data decreases.
In FIG. 2, a method for determining uplink/downlink channels between the Node B and the UE is to establish a downlink channel with some of the time slots used for the downlink transmission among the time slots of the subframe 211 and a specific channelization code, and establish an uplink channel with some of the time slots used for the uplink transmission among the time slots of the subframe 211 and a specific channelization code. The uplink/downlink channels (time slots and channelization codes) established between the UE and the Node B are repeatedly used in a radio frame unit, and when necessary, the uplink/downlink channels (time slots and channelization codes) between the UE and the Node B can be reestablished. A repetition period of the uplink/downlink channels between the UE and the Node B and the number of radio frames, after which the reestablishment is to be performed, may depend upon the type and amount of the data exchanged between the UE and the Node B. In addition, when the uplink transmission data is larger in amount than the downlink transmission data, the uplink transport channel may be repeated more frequently than the downlink transport channel. In FIG. 2, the Kth frame 204 is a frame where the data exchange between the UE and the Node B ends.
FIG. 3 illustrates a typical structure of a time slot in the subframe. The time slot structure illustrated in FIG. 3 may be used for both the uplink transmission and the downlink transmission. Data symbols 311 and 317 are used for transmission of uplink or downlink transmission data, and TFCIs (Transport Format Combination Indicators) 312 and 316 serve to indicate a data rate of a downlink channel transmitted from the Node B to the UE, or data rates of channelization codes for transmitting a downlink channel and the type of data. Further, for the uplink channel, the TFCIs have the same function as in the downlink channel. A midamble 313 is used to identify UEs using the same time slot, or identify downlink channels using the same time slot in the WB-TDD and NB-TDD communication systems. Further, for uplink/downlink transmissions, the midamble 313 is used for channel estimation, and for the downlink transmission, different midambles are used to measure a loss of channel propagation from the Node B to the UE. Alternatively, Node Bs may use different midambles for identification of the Node Bs. A specific sequence is used for the midamble 313, and the number of specific sequences used for the midamble is 128. The channelization codes and the midamble sequences described with reference to FIG. 2 have different characteristics and types, and this will be described for the uplink transmission, with reference to FIG. 3. For the uplink transmission, the channelization code, an orthogonal code, is used for the data symbols 311 and 317, and serves to distinguish data of the UEs, transmitted with the data symbols 311 and 317. A midamble 313 serves to identify which UE is transmitting data, and the midamble 313 is not subject to band expansion with a channelization code.
In addition, SS (Synchronization Shift) 314 is used to transmit a synchronization control command, when synchronization between the UE and the Node B has failed due to a change in the distance between the UE and the Node B during signal transmission, or for other reasons. In response to the command transmitted through the SS 314, the UE can control a transmission point in a ¼-chip unit. Further, GP 318 is a period for separating the current time slot from the next time slot. The GP 318 serves as a guard for preventing interferences between an uplink signal and a downlink signal, when the downlink transmission slot signal is received in the uplink transmission slot period or the uplink transmission slot signal is received in the downlink transmission slot period. Finally, TPC (Transmission Power Control) command 315 is used for controlling downlink transmission power of the Node B, when it is transmitted through the uplink, and the TPC 315 is used for controlling uplink transmission power of the UE, when it is transmitted through the downlink. Herein, a process for determining the TPC command 315 and then transmitting the determined TPC command will be separately described for the uplink transmission power control and the downlink transmission power control.
Uplink Transmission Power Control
In the uplink transmission power control process, the Node B correctly receives data transmitted from the UE through the uplink by controlling a level of the uplink transmission power transmitted by the UE, and controls the uplink transmission power so that the received data does not act as an interference to a signal transmitted from another UE to the Node B due to an excessively high level of the transmission power of the UE. In the NB-TDD communication system, for the uplink transmission power control, the Node B measures a signal-to-interference ratio (SIR) of a signal transmitted over a dedicated physical channel (DPCH) transmitted by the UE, and compares the measured SIR value with a predetermined target SIR value (SIRtarget). In order to correctly receive the data transmitted over the uplink dedicated physical channel of the UE, the Node B transmits an uplink transmission power-down command to the UE, if the measured SIR value is larger than or equal to the target SIR value. However, if the measured SIR value is less than the target SIR value, the Node B transmits an uplink transmission power-up command to the UE. Further, the DPCH is a physical channel for transmitting user data and signaling information from an upper layer, and is used by only the UE designated by the Node B, and the other UEs do no share the DPCH allocated to a specific UE.
Downlink Transmission Power Control
In the downlink transmission power control process, the UE controls transmission power of a downlink signal from the Node B such that the UE can correctly receive the downlink signal without errors. The downlink transmission power control process is performed in the same manner as the uplink transmission power control process. That is, the UE measures SIR of a downlink DPCH signal from the Node B, and compares the measured SIR value with a predetermined target SIR value. As the result of the comparison, if the measured SIR value is less than the target SIR value, the UE transmits an uplink transmission power-up command to the Node B. Otherwise, if the measured SIR value is larger than or equal to the target SIR value, the UE transmits an uplink transmission power-down command to the Node B so that the Node B controls transmission power of a signal transmitted to the UE.
As described with reference to FIGS. 2 and 3, in the NB-TDD communication system, a dedicated channel between the Node B and the UE is allocated through time slot, channelization code and radio frame, and the TPC command is transmitted in a subframe unit. In the NB-TDD communication system, when the uplink/downlink channels between the UE and the Node B are established every subframe, the maximum frequency of transmitting the uplink/downlink TPC commands is 200 Hz; when the uplink/downlink channels between the UE and the Node B are established every radio frame, the maximum frequency of transmissions is 100 Hz; and when the uplink/downlink channels between the UE and the Node B are established every 10 radio frames, the maximum frequency of transmissions is 10 Hz. Further, in the NB-TDD communication system, the uplink/downlink channels between the UE and the Node B may not be equally established: the frequency of establishing the uplink transmission time slot is increased when there is a greater amount of uplink transmission data, and the frequency of establishing the downlink transmission time slot is increased when there is a greater amount of downlink transmission data. Therefore, the frequency of transmitting the uplink TPC command may not be equal to the frequency of transmitting the downlink TPC command.
Accordingly, in the NB-TDD communication system, using only the closed-loop power control method for transmission power control of the DPCH is inefficient in properly setting the uplink/downlink transmission power in the above-stated condition, i.e., in a situation where the uplink/downlink channels for the uplink/downlink transmissions between the UE and the Node B are established unequally or established discontinuously, disturbing smooth transmission of the TPC commands.
In addition, the NB-TDD communication system provides a shared channel in addition to the DPCH as a channel for transmitting user data or signaling information of an upper layer, and the shared channel is equal to the DPCH in a physical structure and a transmission power control method. The shared channel is classified into a downlink shared channel (DSCH) and an uplink shared channel (USCH). In particular, the shared channel can be efficiently used for a packet data service. Unlike the dedicated channel, the shared channel is used by a plurality of UEs by sharing radio resources (e.g., channelization code, time slot and midamble). The packet data is generated on a burst basis (or burstly) in light of its property, and the packet data can be transmitted discontinuously using the shared channel since it is not susceptible to transmission time delay. Therefore, in the case where the transmission power of the shared channel is controlled in the closed-loop power control method as in the DPCH, a period for transmitting the TPC signal may not exist for a long time if a non-transmission period between data blocks on the channel is increased, so that the data may not be normally transmitted.